Method and apparatus for electron density measurement

ABSTRACT

A plasma processing system including a plasma chamber ( 120 ) having a substrate holder ( 128 ) and a monitoring system ( 130 ). The monitoring system ( 130 ) includes a microwave mirror ( 140 ) having a concave surface ( 142 ) located opposite the holder ( 128 ) and a power source ( 160 ) is coupled thereto that produces a microwave signal perpendicular to a wafer plane ( 129 ) of the holder ( 128 ). A detector ( 170 ) is coupled to the mirror ( 140 ) and measures a vacuum resonance voltage of the signal within the chamber ( 120 ). A control system ( 180 ) is provided that measures a first voltage during a vacuum condition and a second voltage during a plasma condition and determines an electron density from a difference between the second voltage and the first voltage. The processing system ( 110 ) can include a plurality of monitoring systems ( 130   a   , 130   b   , 130   c ) having mirrors ( 140   a   , 140   b   , 140   c ) provided in a spatial array located opposite the substrate holder ( 128 ). A method of monitoring electron density in the processing system is provided that includes loading a wafer, setting a frequency of a microwave signal to a resonance frequency, and measuring a first voltage of the signal during a vacuum condition. The method further includes processing the wafer ( 114 ), measuring a second voltage of the signal during a plasma condition, and determining an electron density from a difference between the second voltage and the first voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority and is related to UnitedStates provisional Ser. No. 60/330,555, filed on Oct. 24, 2001. Thepresent application claims priority and is related to United Statesprovisional Ser. No. 60/330,518, filed on Oct. 24, 2001. The contents ofthose applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention generally relates to fabrication ofintegrated circuits in the semiconductor industry.

[0004] 2. Discussion of the Background

[0005] The fabrication of integrated circuits (IC) in the semiconductorindustry typically employs plasma to create and assist surface chemistrywithin a plasma processing chamber necessary to remove material from anddeposit material to a substrate. In general, plasma is formed within theprocessing chamber under vacuum conditions by heating electrons toenergies sufficient to sustain ionizing collisions with a suppliedprocess gas. Moreover, the heated electrons can have energy sufficientto sustain dissociative collisions and, therefore, a specific set ofgases under predetermined conditions (e.g., chamber pressure, gas flowrate, etc.) are chosen to produce a population of charged species andchemically reactive species suitable to the particular process beingperformed within the chamber (e.g., etching processes where materialsare removed from the substrate or deposition processes where materialsare added to the substrate).

[0006] The semiconductor industry is constantly striving to producesmaller ICs and to increase the yield of viable ICs. Accordingly, thematerial processing equipment used to process the ICs have been requiredto meet increasingly more stringent performance requirements for etchingand deposition processes (e.g., rate, selectivity, critical dimension,etc.).

SUMMARY OF THE INVENTION

[0007] The present invention relates to a method and apparatus formonitoring electron density in a plasma processing chamber. The presentinvention advantageously provides a method and apparatus that enablessemiconductor manufacturers to satisfy more stringent performancerequirements for material processing equipment used in the semiconductorindustry.

[0008] The present invention advantageously provides a plasma processingsystem including a plasma chamber having a substrate holder and amonitoring system for use in the plasma chamber. The monitoring systemincludes a microwave mirror provided within the plasma chamber. Themirror has a concave surface being located opposite a substrate holder.The monitoring system further includes a power source coupled to themicrowave mirror, where the power source is configured to produce amicrowave signal extending along an axis generally perpendicular to awafer plane of the substrate holder. The system includes a detectorcoupled to the microwave mirror and configured to measure a vacuumresonance voltage of the microwave signal within the plasma chamber. Themonitoring system also includes a control system connected to thedetector and configured to measure a first voltage during a vacuumcondition and a second voltage during a plasma condition. The controlsystem is configured to determine an electron density from thedifference in the first and second voltages.

[0009] The present invention advantageously provides an alternativeprocessing system that includes a plurality of monitoring systems havingmirrors provided in a spatial array located opposite the substrateholder.

[0010] The present invention further advantageously provides a method ofmonitoring electron density in a plasma chamber. The method utilizes aplasma chamber including a microwave mirror having a concave surfacelocated opposite a substrate holder within the plasma chamber, a powersource coupled to the microwave mirror and configured to produce amicrowave signal extending along an axis generally perpendicular to awafer plane of the substrate holder, and a detector coupled to themicrowave mirror. The method includes the steps of loading a wafer inthe plasma chamber, setting a frequency of a microwave signal outputfrom the power source to a resonance frequency, and measuring a firstvoltage of the microwave signal within the plasma chamber during avacuum condition. The method further includes the steps of processingthe wafer, measuring a second voltage of the microwave signal within theplasma chamber during a plasma condition, and determining the electrondensity from the difference between the first and second voltages.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] A more complete appreciation of the invention and many of theattendant advantages thereof will become readily apparent with referenceto the following detailed description, particularly when considered inconjunction with the accompanying drawings, in which:

[0012]FIG. 1 is a schematic view of an electron density measurementsystem for a plasma processing chamber according to an embodiment of thepresent invention;

[0013]FIG. 2 is a schematic view of an electron density measurementsystem for a plasma processing chamber according to an embodiment of thepresent invention;

[0014]FIG. 3 is an enlarged, exploded, cross-sectional view of amicrowave mirror having an aperture, a microwave window and associatedmounting structure;

[0015]FIG. 4 is a graphical representation of an exemplary cavitytransmission function showing several longitudinal resonances and arespective free spectral range;

[0016]FIG. 5 is a flow diagram of a method of monitoring electrondensity in a plasma processing chamber according to an embodiment of thepresent invention;

[0017]FIG. 6 is a schematic view of a multi-site electron densitymeasurement system for a plasma processing chamber according to analternative embodiment of the present invention;

[0018]FIG. 7 is a schematic view of a multi-site electron densitymeasurement system for a plasma processing chamber according to analternative embodiment of the present invention;

[0019]FIG. 8 is a flow diagram of a method of monitoring electrondensity at multiple sites in a plasma processing chamber according to anembodiment of the present invention; and

[0020]FIG. 9 is a top view of a non-linear mirror configuration for usein a multi-side measurement system according to one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The present invention generally relates to fabrication ofintegrated circuits in the semiconductor industry. The present inventionadvantageously provides a method and apparatus that enablessemiconductor manufacturers to satisfy more stringent performancerequirements for material processing equipment used in the semiconductorindustry.

[0022] A method of improving the performance of material processingequipment is to monitor and control plasma electron density within theprocessing chamber during the manufacturing process. Ideally, the plasmaelectron density is maintained such that the processes being performedare uniformly acting upon the entire surface of the substrate upon whichwork is being performed.

[0023] An exemplary device used to measure plasma electron density is amicrowave system of suitably high frequency to exceed the electronplasma frequency. The device includes at least one reflecting surfaceimmersed in the plasma Microwave power is coupled to a multi-modalresonator (e.g. open resonant cavity) and a detector is utilized tomonitor the transmission of microwave power through the multi-modalresonator. For a Gaussian beam, cavity transmission occurs at discretefrequencies, and the discrete frequencies correspond to an integernumber of half wavelengths between the apex of each mirror, as expressedby the following equation: $\begin{matrix}{{v_{m,n,q} = {v_{0,0,q} = {\frac{c}{2{nd}}\left( {q + \frac{1}{2}} \right)}}},} & (1)\end{matrix}$

[0024] where ν_(0,0q) is a resonant frequency of mode order q (assumingonly longitudinal modes, i.e. m=n=0), c is the speed of light in avacuum, n is the index of refraction for the medium bounded by themirrors and d is the mirror spacing (apex-to-apex) for the multi-modalresonator. For a vacuum, n=1, however, the presence of plasma or, morespecifically, a population of free electrons leads to a reduction of theindex of refraction or an observable increase (shift) of the cavityresonance frequencies ν_(0,0,q). For a given mode q, the shift infrequency can be related to the index of refraction n and, thereafter,the (integrated) electron density <n_(e)>, is expressed by the followingequation: $\begin{matrix}{{{\langle n_{e}\rangle} \cong {\frac{8\pi^{2}ɛ_{o}}{e^{2}}v_{o}\Delta \quad v}},} & (2)\end{matrix}$

[0025] for ν_(o)>>ω_(pe)/2π. For further details, the use of the abovesystem to measure plasma electron density is described in InternationalApp. No. PCT/US00/19539 (based upon U.S. Ser. No. 60/144,880),International App. No. PCT/US00/19536 (based upon U.S. Ser. No.60/144,883), International App. No. PCT/US00/19535 (based upon U.S. Ser.No. 60/144,878), and International App. No. PCT/US00/19540 (based uponU.S. Ser. No. 60/166,418), each of which is incorporated herein byreference in their entirety.

[0026] An apparatus is now described that enables real-time spatialresolution of the electron density. In an embodiment depicted in FIG. 1,a monitoring system 130 is aligned substantially perpendicular to thewafer plane 129 wherein a first reflecting surface is immersed in theplasma within an upper wall opposite a second reflecting surface. Themonitoring system 130 can be, for example, a multi-modal resonator. Thefirst reflecting surface can be, for example, a microwave mirror 140 andthe second reflecting surface can be, for example, a substrate 114and/or substrate holder 128.

[0027] An embodiment of a plasma processing system 110 as depicted inFIG. 1 includes a plasma chamber 120 and a monitoring system 130 for usein the plasma chamber 120. The monitoring system generally includes amicrowave mirror 140, a power source 160, a detector 170, and a controlsystem 180. The plasma chamber 120 generally includes a base wall 122,an upper wall 124, and side-walls including a first side wall 126 and asecond side wall 127. The plasma chamber 120 also includes a substrateholder (or chuck assembly) 128 having a wafer plane 129, such as anupper surface of the substrate holder 128 upon which a substrate 114 ispositioned in order to be processed within the plasma chamber 120.

[0028] The microwave mirror 140 can have, for example, a concave surface142 and is provided within the plasma chamber 120. In the embodimentdepicted in FIG. 1, the mirror 140 is integrated within the upper wall124 of the plasma chamber 120. The concave surface 142 of the microwavemirror 140 is oriented opposite the substrate holder 128.

[0029] The power source 160 is coupled to the microwave mirror 140 andis configured to produce a microwave signal. The microwave signal ormicrowave beam 145 produced by the power source 160 extends in adirection generally perpendicular to a wafer plane 129 of a substrateholder 128 adapted to be provided within the plasma chamber 120. Theembodiment of the monitoring system 130 depicted in FIG. 1 also includesthe detector 170 coupled to the microwave mirror 140. The detector 170is configured to measure a voltage related to the microwave signalwithin the plasma chamber 120. The control system 180 is connected tothe detector 170 and is configured to measure a first voltage during avacuum condition, measure a second voltage during a plasma condition,and determine an electron density from the difference between the firstand second measured voltages. The control system 180 that includes alock-on circuit 182 connected to the power source 160 and the detector170, and can additionally include a computer connected to the lock-oncircuit 182.

[0030] The upper wall 124 of the chamber 120 includes a waveguideaperture 144 configured to couple the power source 160 to the microwavemirror 140, and a detector aperture 146 configured to couple thedetector 170 to the microwave mirror 140. Microwave window assemblies190 each including a microwave window 192 are provided for both thewaveguide aperture 144 and the detector aperture 146. The microwavewindow assemblies 190 are identical in structure to the microwave windowassembly depicted in FIG. 3 to be described below. The microwave windowsare implemented to maintain vacuum integrity. Alternately, separatemirrors can be provided for the power source 160 and the detector 170.

[0031] In an alternate embodiment as depicted in FIG. 2, the upper wall224 of process chamber 120 can comprise an inner domed surface withinwhich a monitoring system 130 can be formed. In an alternate embodiment,the chamber 120 can be frustoconical.

[0032]FIG. 3 depicts a detailed schematic of a microwave window assembly190 for mirror 140, which is used to provide a coupling from the powersource 160 through aperture 144. A window assembly 190 having anidentical structure is preferably provided for the second aperture 146in mirror 140, which is used to provide a coupling to the detector 170.

[0033] The microwave window assembly 190 depicted in FIG. 3 includes amicrowave window 192 that is fastened between a window flange 194 and arear surface 147 of the microwave mirror 140. In the embodiment depictedin FIG. 3, the window 192 is provided within a recessed portion 148 onthe rear surface 147 of microwave mirror 140. The microwave window 192is provided between a first O-ring 196 provided on the window flange 194and a second O-ring 197 provided on the rear surface 147 of microwavemirror 140. A plurality of fasteners 198 are provided to mechanicallyconnect the window flange 194 to microwave mirror 140 such that themicrowave window 192 is securely mounted to the rear surface 147 ofmicrowave mirror 140. The window 192 is centered on a waveguide aperture195 extending through the window flange 194 and the waveguide aperture144 extending through microwave mirror 140. The rectangular waveguideapertures 144 and 195 are sized for a specific microwave band ofoperation and are fabricated using EDM (electrical discharge machining).The microwave window 192 is fabricated from a dielectric material suchas alumina, sapphire, aluminum nitride, quartz, polytetrafluoroethylene(PTFE/Teflon), or Kapton. The window 192 is preferably fabricated fromsapphire due to its compatibility with the oxide etch processes.

[0034] The microwave mirror 140 is preferably fabricated from aluminum.In alternative embodiments, microwave mirror 140 is anodized withpreferably a 10 to 50 micron thick anodization or coated with a materialsuch as Yttria (y₂O₃).

[0035] The microwave power source 160 is preferably an electronicallytunable voltage controlled Gunn diode oscillator (VCO). When the VCO isbiased with a direct current voltage, the output frequency can be variedover some spectral range. Therefore, the VCO specifications generallyinclude center frequency, bandwidth and minimum output power. In orderto facilitate the use of the above-described system, it is preferredthat the VCO bandwidth is at least comparable to the free spectral range(FSR). For example, at 35 GHz, a commercially available VCO is aWBV-28-20160RI Gunn diode oscillator offered by Millitech, LLC (20Industrial Drive East, South Deerfield, Mass. 01373-0109). Thespecifications for this VCO include a center frequency of 35 GHz withplus or minus 1 GHz bandwidth and a minimum output power of 40 mW. A 2GHz bandwidth can be suitable for a spacing (between the upper wall 20and wafer 35) of approximately 7.5 cm. The bias tuning range cangenerally extend from +25 V to −25 V, thereby adjusting this biasvoltage leads to a change in the output frequency of the VCO. Inalternate embodiments, operation at higher frequencies, such as 70 GHzand 105 GHz, can be achieved using a frequency doubler (MUD-15-16F00) ortripler (MUT-10-16F00) with the above mentioned VCO. Using the aboveconfiguration, a center frequency of 70 GHz with plus or minus 2 GHzbandwidth and a minimum output power of 0.4 to 0.9 mW and a centerfrequency of 105 GHz with plus or minus 3 GHz bandwidth and a minimumoutput power of 0.4 to 0.7 mW can be achieved, respectively. In anadditional embodiment, a 94 GHz VCO (Model GV-10) is used and iscommercially available from Farran Technology LTD (Ballincollig, Cork,Ireland). The Model GV-10 VCO has a center frequency of 94 GHz with plusor minus 750 MHz bandwidth, a minimum output power of 10 mW, and avaractor tuning range of 0 to −25 V. For small mirror spacing (i.e. <5cm), a microwave input with sufficient power and large bandwidth couldbe required. In one embodiment, an active multiplier chain is utilizedwith a low frequency microwave oscillator to achieve bandwidths as greatas plus or minus 12 GHz. For example, an active multiplier chain for usein the range of 75 to 100 GHz is a Model AMC-10-R000that is commerciallyavailable from Millitech, LLC. In general, the power should besufficiently high to achieve a usable signal-to-noise ratio for thediagnostic, however, the power should not be increased above tens ofmilliwatts in order to avoid wafer damage.

[0036] The detector 170 is preferably a general purpose diode detectorsuch as those commercially available from Millitech, LLC. For example, aDXP-15-RNFW0 and a DXP-10-RNFW0 are general purpose detectors in theV-band (50 to 75 GHz) and W-band (75 to 110 GHz), respectively.

[0037] The embodiment of the present invention depicted in FIG. 1 has acontrol system 180 that includes a lock-on circuit 182 connected to thepower source 160 and the detector 170, and a computer 184 connected tothe lock-on circuit 182. The lock-on circuit 182 can be utilized to lockthe frequency of the microwave signal output from the microwave powersource 160 to a pre-selected cavity resonance. The lock-on circuit 182superposes a dither signal (e.g. 1 kHz, 10 mV amplitude square wave) ona direct current voltage substantially near the voltage and relatedoutput VCO frequency that corresponds with a pre-selected longitudinalfrequency in the monitoring system 130 of FIG. 1 (i.e. when the outputfrequency falls within the resonance envelope, an error signal can beproduced to move the output frequency of the VCO to the frequencyassociated with the resonance peak). The signal detected by the detector170 is provided to the lock-on circuit 182, where it represents a firstderivative of the cavity transmission function (transmitted power versusfrequency). The signal input to the lock-on circuit 182 from thedetector 170 provides an error signal by which the direct currentcomponent of the VCO bias voltage is adjusted to drive the VCO outputfrequency to the frequency associated with the peak of a pre-selectedlongitudinal resonance as shown in FIG. 4. FIG. 4 presents an exemplarycavity transmission function (from a negative polarity detector)indicating several longitudinal resonances and the respective freespectral range (FSR). The cavity transmission as shown in FIG. 4 can beobtained by sweeping the VCO across a suitable frequency rangesufficiently greater than the FSR.

[0038] As described above, the introduction of plasma within the chamber120 causes a shift in frequency for each of the resonances shown in FIG.4 (i.e. each of the resonances shift to the right in FIG. 4 when theelectron density is increased or the index of refraction is decreasedaccording to equation (1)). Therefore, once the output frequency of theVCO is locked to a selected cavity resonance, the direct current biasvoltage with and without plasma can be recorded and the frequency shiftof the selected resonance is determined from the voltage difference andthe respective VCO calibration. For example, in wafer processing, thedirect current bias voltage is recorded once a new wafer is received bythe process tool for materials processing and prior to the ignition ofplasma Hereinafter, this measurement will be referred to as the vacuumresonance voltage. Once the plasma is formed, the direct current biasvoltage is obtained as a function of time for the given wafer and thetime varying voltage difference or ultimately electron density (viaequation (2)) is recorded.

[0039]FIG. 5 is a flowchart of a method of monitoring the bias voltagerepresentative of electron density from wafer-to-wafer utilizing thesystems described in FIGS. 1 and 2. The process begins with a step 200of loading a wafer and preparing the chamber for process conditions(i.e. evacuating the chamber, initiating gas flow, etc.). Once the waferis loaded, a cavity resonance is selected and the lock-on circuit isprogrammed to lock the VCO output frequency to the selected resonantfrequency. The VCO bias voltage corresponding to the pre-selectedresonance during a vacuum condition is measured in step 202 and theprocess proceeds according to a process recipe stored on the processcomputer in step 204. During the process in step 206, a second VCO biasvoltage under a plasma condition is measured as a function of time, adifference between the second VCO bias voltage and the first VCO biasvoltage is computed as a function of time, an electron density isdetermined from the voltage difference per equation (2), and theelectron density is displayed through a graphical user interface (GUI)as a function of time during the process. The measurements of steps 202and 206 can be, for example, a single signal comprising the measuredvoltage as a function of time. When the process is complete, the statusof the batch is evaluated in step 212. If the batch is incomplete, anext wafer is processed in step 208 and steps 200 through 212 arerepeated. If the batch is complete in step 212, a subsequent batch canbe processed.

[0040] The present invention provides a method of monitoring electrondensity in a plasma chamber, such as that depicted in FIGS. 1 and 2. Forexample, the plasma chamber 120 includes a microwave mirror 140 having aconcave surface 142 located opposite a substrate holder 128 within theplasma chamber 120, a power source 160 coupled to the microwave mirror140 and configured to produce a microwave signal extending along an axisgenerally perpendicular to a wafer plane 129 of the substrate holder128, and a detector 170 coupled to the microwave mirror 140. The methodof the present invention includes the steps of loading a wafer 114 inthe plasma chamber 120, setting a frequency of a microwave signal outputfrom the power source 160 to a resonance frequency, and measuring afirst voltage of the microwave signal during a vacuum condition withinthe plasma chamber 120 using the detector 170. The method furtherincludes the steps of processing the wafer 114, measuring a secondvoltage of the microwave signal during a plasma condition within theplasma chamber 120 using the detector 170, and determining an electrondensity (per equation (2)) from a difference between the second measuredvoltage and the first measured voltage.

[0041] The configuration described above and depicted in FIGS. 1, 2, 3and 5 enables the measurement of the integrated electron density in amonitoring system 130 within the influence of the microwave beam. Inaddition to monitoring the integrated plasma density at a single regionabove substrate 114, an alternate embodiment can be configured tomonitor the plasma density at more than one location above substrate114. The process uniformity which is strongly affected by the uniformityof the plasma density is critical in achieving maximum yield and qualityof devices across an entire 200 mm to 300 mm wafer (and larger).

[0042] In an alternate embodiment as depicted in FIG. 6, a plurality ofmonitoring systems 130 a, 130 b, and 130 c substantially identical tothose described above are employed with respective mirrors 140 a, 140 b,and 140 c to achieve spatially resolved electron density measurements.The plurality of monitoring systems 130 a, 130 b, and 130 cincludemicrowave mirrors 140 a, 140 b, and 140 c that are provided in a spatialarray located opposite the substrate holder 128. The monitoring systemsof such an array can be operated by simultaneously using the method ofmonitoring electron density in a plasma chamber as depicted in FIG. 1.In such a configuration, the electron density can be determined atmultiple sites above the substrate 114, and these measurements can be,for example, correlated with the process performance parameters (i.e.spatial distribution of etch rate, etch selectivity, etc.). Themulti-site measurement of electron density can ultimately be employed todiagnose the uniformity of a process.

[0043] In the embodiment depicted in FIG. 6, a linear array of mirrorsis provided, however, other configurations of the mirror array can beutilized to provide for an even distribution of monitoring systems abovethe substrate holder 128, as discussed later with respect to FIG. 9.

[0044] In an alternate embodiment as depicted in FIG. 7, the upper wall224 of process chamber 120 can be curved; a plurality of monitoringsystems 130 a-c can be formed within the curved wall. In an alternateembodiment, the chamber 120 can be frustoconical.

[0045]FIG. 8 is a flowchart of a second method of monitoring the biasvoltage representative of electron density from wafer-to-wafer utilizingthe system described in FIG. 5. The process begins with a step 300 ofloading a wafer and preparing the chamber for process conditions (i.e.evacuating the chamber, initiating gas flow, etc.). Once the wafer isloaded, a cavity resonance is selected and the lock-on circuit isprogrammed to lock the VCO output frequency to the selected resonantfrequency for each multimodal resonator 130(a-c) in FIGS. 6 and 7. TheVCO bias voltage corresponding to the pre-selected resonance during avacuum condition is measured in step 302 (i.e. for each multi-modalresonator 130a-c) and the process proceeds according to a process recipestored on the process computer in step 304. During the process in step306, a second VCO bias voltage under a plasma condition is measured as afunction of time, a difference between the second VCO bias voltage andthe first VCO bias voltage is computed as a function of time, anelectron density is determined from the voltage difference per equation(2), and the electron density is displayed through a graphical userinterface (GUI) as a function of time during the process. Step 306 isrepeated for each multi-modal resonator (130 a-c) in FIGS. 6 and 7. Themeasurements of steps 302 and 306 can be, for example, a single signalcomprising the measured voltage as a function of time. At the completionof processing for a given wafer, the uniformity of the electron densityis computed and a determination of whether the uniformity is withinprescribed limits is made in step 314. If the uniformity of electrondensity exceeds the prescribed limit, then an operator is notified instep 220. When the process is complete, the status of the batch isevaluated in step 312. If the batch is incomplete, a next wafer isprocessed in step 308 and steps 300 through 314 are repeated. If thebatch is complete in step 312, a subsequent batch can be processed instep 316.

[0046]FIG. 9 is a top view of a multi-side monitoring system. While anarray of seven sites is shown, more or fewer sites can be used. Thearray can be non-linear (as shown in FIG. 9) or linear (as shown inFIGS. 6 and 7). The spacing between sites can be uniform or non-uniformand may vary with radius.

[0047] As an alternative to the processes depicted in FIGS. 5 and 8, theprocessing of a batch can be terminated mid-batch if the uniformity isnot within prescribed limits. In such an embodiment, the system trackswhich wafers still need to be processed when the wafer cartridge isreloaded.

[0048] It should be noted that the exemplary embodiments depicted anddescribed herein set forth the preferred embodiments of the presentinvention, and are not meant to limit the scope of the claims hereto inany way.

[0049] Numerous modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

1. A monitoring system for use in a plasma chamber, said monitoringsystem comprising: a first reflecting surface, said first reflectingsurface arranged opposite a second reflecting surface, wherein one ofsaid first and second reflecting surfaces is adapted to be providedwithin the plasma chamber; a power source coupled to said firstreflecting surface, said power source being configured to produce amicrowave signal extending along an axis generally perpendicular to awafer plane; a detector coupled to said first reflecting surface, saiddetector being configured to measure at least one signal related to saidmicrowave signal within the plasma chamber; and a control systemconnected to said detector and configured to process said at least onesignal, said control system being configured to determine an electrondensity from said at least one signal.
 2. The monitoring systemaccording to claim 1, wherein said first reflecting surface is amicrowave mirror, said microwave mirror comprising a concave surface. 3.The monitoring system according to claim 1, wherein said secondreflecting surface is at least one of a substrate, a chamber wall, and asubstrate holder.
 4. The monitoring system according to claim 1, whereinsaid power source is coupled to said first reflecting surface via anaperture in said first reflecting surface.
 5. The monitoring systemaccording to claim 1, further comprising: a first microwave windowprovided between said power source and said first reflecting surface;and a second microwave window provided between said detector and saidfirst reflecting surface.
 6. The monitoring system according to claim 3,further comprising a flange configured to mount said first microwavewindow to a rear surface of said first reflecting surface.
 7. Themonitoring system according to claim 3, wherein said first microwavewindow and said second microwave window comprise a dielectric material.8. The monitoring system according to claim 3, wherein said firstmicrowave window and said second microwave window comprise at least oneof alumina, sapphire, aluminum nitride, quartz, polytetrafluoroethylene,and Kapton.
 9. The monitoring system according to claim 1, wherein saidpower source comprises a voltage controlled oscillator.
 10. Themonitoring system according to claim 1, wherein said detector comprisesa diode detector.
 11. The monitoring system according to claim 1,wherein said first reflecting surface comprises aluminum.
 12. Themonitoring system according to claim 1, wherein said first reflectingsurface comprises an anodized surface.
 13. The monitoring systemaccording to claim 12, wherein said anodized surface is anodized with ananodization having a thickness in a range from 10 to 50 microns.
 14. Themonitoring system according to claim 1, wherein said first reflectingsurface comprises a Yttria coating.
 15. The monitoring system accordingto claim 1, wherein said control system comprises a lock-on circuitconfigured to lock a frequency of said microwave signal to apre-selected resonance frequency, said lock-on circuit being configuredto receive a detection signal from said detector and provide acorresponding error signal to said power source to adjust an outputfrequency of said microwave signal to a frequency associated with a peakof a longitudinal resonance.
 16. The monitoring system according toclaim 15, wherein said control system further comprises a computerconnected to said lock-on circuit.
 17. A plasma processing systemcomprising: a plasma chamber having a substrate holder; and a monitoringsystem for use in said plasma chamber, said monitoring systemcomprising: a microwave mirror having a concave surface, said microwavemirror provided within the plasma chamber with said concave surfacebeing located opposite said substrate holder, a power source coupled tosaid microwave mirror, said power source being configured to produce amicrowave signal extending along an axis generally perpendicular to awafer plane of said substrate holder, a detector coupled to saidmicrowave mirror, said detector being configured to measure a vacuumresonance voltage of said microwave signal within the plasma chamber,and a control system connected to said detector and configured tomeasure a first voltage during a vacuum condition and a second voltageduring a plasma condition, said control system being configured todetermine an electron density from the difference between said secondvoltage and said first voltage.
 18. The plasma processing systemaccording to claim 17, wherein said power source is coupled to saidmicrowave mirror via an aperture in said microwave mirror.
 19. Theplasma processing system according to claim 17, further comprising: afirst microwave window provided between said power source and saidmicrowave mirror; and a second microwave window provided between saiddetector and said microwave mirror.
 20. The plasma processing systemaccording to claim 17, wherein said control system comprises a lock-oncircuit configured to lock a frequency of said microwave signal to apre-selected resonance frequency, said lock-on circuit being configuredto receive a detection signal from said detector and provide acorresponding error signal to said power source to adjust an outputfrequency of said microwave signal to a frequency associated with alongitudinal resonance.
 21. A plasma processing system comprising: aplasma chamber having a substrate holder; and a plurality of monitoringsystems for use in said plasma chamber, said monitoring systems eachcomprising: a microwave mirror having a concave surface, said microwavemirror provided within the plasma chamber with said concave surfacebeing located opposite said substrate holder, a power source coupled tosaid microwave mirror, said power source being configured to produce amicrowave signal extending along an axis generally perpendicular to awafer plane of said substrate holder, a detector coupled to saidmicrowave mirror, said detector being configured to measure a vacuumresonance voltage of said microwave signal within the plasma chamber,and a control system connected to said detector and configured tomeasure a first voltage during a vacuum condition and a second voltageduring a plasma condition, said control system being configured todetermine an electron density from difference between said secondvoltage and said first voltage.
 22. The plasma processing systemaccording to claim 21, wherein said plurality of monitoring systemscomprise microwave mirrors provided in a spatial array located oppositesaid substrate holder.
 23. The plasma processing system according toclaim 21, wherein said power source is coupled to said microwave mirrorvia an aperture in said microwave mirror.
 24. The plasma processingsystem according to claim 21, further comprising: a first microwavewindow provided between said power source and said microwave mirror; anda second microwave window provided between said detector and saidmicrowave mirror.
 25. The plasma processing system according to claim21, wherein said control system comprises a lock-on circuit configuredto lock a frequency of said microwave signal to a pre-selected resonancefrequency, said lock-on circuit being configured to receive a detectionsignal from said detector and provide a corresponding error signal tosaid power source to adjust an output frequency of said microwave signalto a frequency associated with a peak of a pre-selected longitudinalresonance.
 26. A monitoring system for use in a plasma chamber, saidmonitoring system comprising: a microwave mirror having a concavesurface, said microwave mirror adapted to be provided within the plasmachamber with said concave surface being located opposite a substrateholder; means for producing a microwave signal extending along an axisgenerally perpendicular to a wafer plane of the substrate holder, saidmeans for producing a microwave signal being coupled to said microwavemirror; means for measuring a voltage of said microwave signal withinthe plasma chamber; and means for determining a difference between afirst voltage during a vacuum condition and a second voltage during aplasma condition and determining an electron density from saiddifference.
 27. The monitoring system according to claim 26, whereinsaid means for determining comprises a means for locking a frequency ofsaid microwave signal to a pre-elected resonance frequency, said meansfor locking being configured to receive a detection signal from saidmeans for measuring and provide a corresponding error signal to saidmeans for producing a microwave signal to adjust an output frequency ofsaid microwave signal to a frequency associated with a peak of apre-selected longitudinal resonance.
 28. A plasma processing systemcomprising: a plasma chamber having a substrate holder; and a monitoringsystem for use in said plasma chamber, said monitoring systemcomprising: a microwave mirror having a concave surface, said microwavemirror provided within said plasma chamber with said concave surfacebeing located opposite said substrate holder; means for producing amicrowave signal extending along an axis generally perpendicular to awafer plane of said substrate holder, said means for producing amicrowave signal being coupled to said microwave mirror; means formeasuring a voltage of said microwave signal within said plasma chamber;and means for determining a difference between a first voltage during avacuum condition and a second voltage during a plasma condition anddetermining an electron density from said difference.
 29. The plasmaprocessing system according to claim 28, wherein said means fordetermining comprises a means for locking a frequency of said microwavesignal to a pre-selected resonance frequency, said means for lockingbeing configured to receive a detection signal from said means formeasuring and provide a corresponding error signal to said means forproducing a microwave signal to adjust an output frequency of saidmicrowave signal to a frequency associated with a peak of a pre-selectedlongitudinal resonance.
 30. A plasma processing system comprising: aplasma chamber having a substrate holder; and a plurality of monitoringsystems for use in said plasma chamber, said monitoring systems eachcomprising: a microwave mirror having a concave surface, said microwavemirror provided within said plasma chamber with said concave surfacebeing located opposite said substrate holder; means for producing amicrowave signal extending along an axis generally perpendicular to awafer plane of said substrate holder, said means for producing amicrowave signal being coupled to said microwave mirror; means formeasuring a voltage of said microwave signal within said plasma chamber;and means for determining a difference between a first voltage during avacuum condition and a second voltage during a plasma condition anddetermining an electron density from said difference.
 31. The plasmaprocessing system according to claim 30, wherein said plurality ofmonitoring systems comprise microwave mirrors provided in an spatialarray located opposite said substrate holder.
 32. The plasma processingsystem according to claim 30, wherein said means for determiningcomprises a means for locking a frequency of said microwave signal to apre-selected resonance frequency, said means for locking beingconfigured to receive a detection signal from said means for measuringand provide a corresponding error signal to said means for producing amicrowave signal to adjust an output frequency of said microwave signalto a frequency associated with a peak of a pre-selected longitudinalresonance.
 33. A method of monitoring electron density in a plasmachamber, the plasma chamber including a first reflecting surface locatedopposite a second reflecting surface within the plasma chamber, a powersource coupled to said first reflecting surface and configured toproduce a microwave signal extending along an axis generallyperpendicular to a wafer plane of the substrate holder, and a detectorcoupled to said second reflecting surface, said method comprising thesteps of: loading a wafer in the plasma chamber; setting a frequency ofa microwave signal output from the power source to a resonancefrequency; measuring a first voltage of the microwave signal within theplasma chamber during a vacuum condition; processing the wafer;measuring a second voltage of the microwave signal within the plasmachamber during a plasma condition; and determining an electron densityfrom a difference between said second voltage and said first voltage.34. The method according to claim 33, wherein the plasma chamber furtherincludes at least one additional surface located opposite the substrateholder within the plasma chamber, at least one additional power sourcecoupled to the at least one additional reflecting surface and configuredto produce an additional microwave signal extending along an axisgenerally perpendicular to a wafer plane of the substrate holder, and atleast one detector coupled to the at least one additional reflectingsurface, and wherein said method further comprises the steps of: settinga frequency of a microwave signal output from the at least one powersource to a resonance frequency; measuring a first additional voltage ofthe additional microwave signal within the plasma chamber during avacuum condition; measuring a second additional voltage of theadditional microwave signal within the plasma chamber during a plasmacondition; and determining an additional electron density from anadditional difference between said second additional voltage and saidfirst additional voltage.
 35. The method according to claim 34, whereinsaid method further comprises the steps of: determining a uniformity ofsaid electron density and said additional electron density; andcomparing said uniformity to a prescribed limit.
 36. The methodaccording to claim 34, wherein said method further comprises the stepof: notifying an operator if said uniformity exceeds said prescribedlimit.
 37. The method according to claim 34, wherein the firstreflecting surface and the at least one additional reflecting surfaceare provided in a spatial array located opposite said substrate holder.38. The method according to claim 33, further comprising the steps of:locking a frequency of the microwave signal to a pre-selected resonancefrequency; receiving a detection signal from the detector; and providinga corresponding error signal to the power source to adjust an outputfrequency of the microwave signal to a frequency associated with a peakof a pre-selected longitudinal resonance.